The present invention relates to a sub-harmonic generator for producing a synthesized signal derived from an input signal but including energy levels at frequencies not contained in the input signal, and the present invention also relates to an expansion processor for increasing the stereo width produced by signals from left and right channels.
Conventional sub-harmonic generators are used to modify an input signal to produce a sub-harmonic signal having at least some desirable characteristics. In music reproduction/processing contexts, an input signal may include frequency components taken from an audible range of about 20 Hz to about 20,000 Hz. The conventional sub-harmonic generator produces an output signal that includes energy at substantially all of the frequency components of the input signal plus additional energy at frequency components in a sub-harmonic range. In some cases, the output signal includes energy at only a subset of the frequency components of the input signal (such as a sub-woofer range) plus the additional energy in the sub-harmonic range. Usually, a range of frequency components from the input signal are utilized to derive the frequency components in the sub-harmonic range, and the input signal is augmented with the frequency components in the sub-harmonic range to obtain the output signal.
In theory, these conventional sub-harmonic generators produce desirable characteristics in the output signal, such as increased signal energy in the sub-harmonic range, thereby producing a richer base response when converted into audible sound energy. In practice, however, the audible characteristics of the output signal from conventional sub-harmonic generators suffer from a number of disadvantages, namely (i) a relatively flat (or “cardboard”) audible sound is obtained from the output signal due primarily to the increase in energy from sub-harmonic frequency components without modifying other frequency characteristics of the input signal, this disadvantage may also manifest in a “rumbly” sound depending on the frequency content of the input signal; and (ii) the audible sound exhibits poor “attack” and “decay” characteristics due to an inability by the sub-harmonic generator to accurately reflect an amplitude envelope of the input signal as a function of the frequency components of interest. Thus, the energy of the output signal in the sub-harmonic frequency range does not exhibit desirable amplitude characteristics. In addition, conventional sub-harmonic generators have not effectively utilized sub-harmonic signals in stereo applications, particularly where maintaining stereo “width” is of importance.
It would be desirable to obtain a new sub-harmonic generator that avoids flat, cardboard sounding characteristics in an output signal by modifying frequency components at least partially outside the sub-harmonic range. It would also be desirable to obtain a sub-harmonic generator that exhibits superior attack and decay characteristics, preferably by using the amplitude envelope of the input signal (as a function of frequency components in the relevant frequency range) in producing the output signal. It is also desirable to obtain an expansion processor for increasing stereo width characteristics created by signals from left and right channels, particularly where sound clarity is improved above certain frequencies.